Ubuntu is a free Linux distribution, based on Debian with the Gnome desktop environment. I've made a more detailed explanation of how Ubuntu works here. Releases normally come out every half year (barring delays), are supported for 18 months (longer for "Long Term Support" (LTS) releases) and are numbered by the year and month as well as given an alliterative animal name:

The name "Grumpy Groundhog" was also reserved, for a special version that "will never actually be released, instead it will be in a state of perpetual development, representing the very cutting edge of upstream and distro packaging." I don't know if anything ever actually became of it though...
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